f/stop differences between 4x5 and 8x10 lenses

[Dan_5988]

Aside from the massive cut off i get when using my schneider 4x5 150mm xenar on my 8x10, do I need to correct for the f/stop in anyway? I notice my 300mm lens at 5.6 is much brighter than my 150mm at 4.5 in the center of the ground glass...that doesnt make any sense does it?

[Donald Qualls]

The f/stop is "relative aperture" -- it's the lens opening divided by focal length. Since the focal length is the same regardless of the film you're projecting onto, no correction is needed based on format.

However: your 4x5 lenses will probably nicely cover 8x10 at macro distances (a 150 mm at 1:1 has the same coverage as a similar design 300 mm at infinity), but when focused close, you *do* need to correct your exposure to account for the longer effective focal length; at 1:2 (image half life size) you need to add one stop, and at 1:1 you need to add two stops. This is because the aperture opening, the same size at 1:1 as at infinity, is correctly divided by the doubled effective focal length in caclulating exposure at 1:1. In effect, if the scale reads f/32, you're really at f/64 when focused at 1:1.

There's no sensible reason for your 300 mm to be brighter than your 150, if both are in focus, unless you have a Fresnel in your focusing screen with a Fresnel focal length that's much closer to 300 mm than 150 mm (I'd expect any factory installed Fresnel in an 8x10 to fulfill that condition, BTW, since 300 mm is a normal lens in that format); in that case, it would intensify the light from the 300 mm much more effectively than the light (which, wide open, should be a half stop brighter at f/4.5 vs. f/5.6) from the 150 mm.

[Ole Tjugen]

With the higher angle between incoming light rays and the ground glass - at the same position on the GG - it's to be expected that the 150 will look dimmer than the 300 off-axis. With both focused at infinity, the 150/4.5 should be2/3 of one stop brighter in the center than the 300/5.6.

That's the same as the "bright spot" when using wide angle lenses: It's not the light falloff, it's the angle of incidence.